As a general rule, when you encounter an issue, always make sure that you use the most recent version of Orthanc.

Also make a search on the Orthanc Users discussion forum, and make a search in the present Orthanc Book (there is a search field at the top of this page). Your issue might indeed have already been discussed in the past or in the FAQ.


  • If Orthanc fails to start with the error “The TCP port of the DICOM server is privileged or already in use”, this means another software is already using the port Orthanc is trying to use. Usually, this means that an other instance of Orthanc is running. However, note that, by default, Orthanc uses port 4242 which might also be used by other software like a Juniper VPN client. To determine which other process is using the port:

    On Windows, you may use the Resource Monitor. In the Network tab, check the Listening Ports.

    On Linux, you may use this command line: sudo ss --tcp --listen --numeric --processes.

    Starting with version 1.3.0, the check at Orthanc startup is more robust (it also checks for UDP socket using the same port) and Orthanc 1.3.0 might display error messages that where not displayed by previous versions.

  • If Orthanc does not start anymore after a hard shutdown and if you use the Orthanc Web viewer plugin, this might reflect a corruption in the cache of the Web viewer. In such a case, it is safe to remove the folder that contains the cache. By default, this folder is called OrthancStorage/WebViewerCache/ (cf. configuration option CachePath in the WebViewer section). Of course, don’t remove the folder OrthancStorage/, as it contains the DICOM files.

DICOM networking

Validating DICOM files

  • Invalid DICOM files are often encountered in practice. Such files can cause failures in Orthanc, or can prevent DICOM network transfers. You can validate DICOM files by using the dciodvfy command-line tool (cf. its documentation) from the dicom3tools project by David Clunie.

    The core team of Orthanc will only provide support for DICOM files that are reported as valid by dciodvfy.

  • Side-note: The default transfer syntax of DICOM is Little Endian Implicit (1.2.840.10008.1.2). For DICOM files that include private tags, we recommend using Little Endian Explicit (1.2.840.10008.1.2.1) instead Little Endian Implicit whenever possible. Instead, in Little Endian Explicit, each DICOM tag has an explicit declaration of its value representation (type), which contrasts with Little Endian Implicit that necessitates to configure the dictionary of private tags to be properly handled in some operations (cf. the Dictionary configuration option).

Orthanc Explorer

  • I cannot login to Orthanc Explorer: For security reasons, access to Orthanc from remote hosts is disabled by default. Only the localhost is allowed to access Orthanc. You have to set the RemoteAccessAllowed option in the configuration file to true. It is then strongly advised to set AuthenticationEnabled to true and to add a user to the RegisteredUsers option, also in the configuration file.

Performance issues

  • Run-time debug assertions: If performance is important to you, make sure to add the option -DCMAKE_BUILD_TYPE=Release when invoking cmake while compiling Orthanc. Indeed, by default, run-time debug assertions are enabled, which can seriously impact performance, especially if your Orthanc server stores a lot of DICOM instances.

    Note that the official Docker images of Orthanc <= 1.0.0 were not compiled in Release mode. As a consequence, to improve performance, make sure to use either the mainline version of the container (run docker pull jodogne/orthanc to ensure you use the most recent version of the mainline), or versions more recent than jodogne/orthanc:1.1.0.

  • Orthanc slows down if storing many files: The default database engine that is built in Orthanc is SQLite. As SQLite is above all a lightweight database engine, it is not designed to store very large datasets. If you are sure that you have properly disabled run-time debug assertions (cf. above), but still experience degradation in performance over time, you should seriously consider switching to a more scalable database engine. To this end, you can notably check out the official PostgreSQL plugin.

    As a rule of thumb, the performance of the default SQLite engine built in Orthanc should run fine up to about 50,000 DICOM instances. However, we have seen Orthanc running fairly well with more than 2 millions of instances. This limit really depends upon your application, and upon the patterns of access to the database.

    Also check out the section about the Scalability of Orthanc.

  • Orthanc Explorer is slow under Windows on the localhost:

    • Favor instead of localhost when specifying the network address of a server. Users have reported that this minor change can massively improve performance on Windows. Starting with Orthanc 1.0.1, the samples from the source distribution have been adapted in this way.

    • As an alternative, you can disable IPv6 support. This is a Windows-specific problem that is discussed here and here.

  • If you experience slow DICOM transfers under GNU/Linux, please read the following bug report. This issue does not affect all the versions of GNU/Linux. A patch to this issue is shipped with the Orthanc source code. In order to take advantage of this patch, you need to statically link Orthanc against DCMTK by using the -DUSE_SYSTEM_DCMTK=OFF flag when invoking CMake.

Checking integrity of the storage area

Orthanc stores, in its database, an MD5 hash of the files stored in its storage area (which notably includes the DICOM files), provided that the StoreMD5ForAttachments configuration option is set to true.

This MD5 corresponds to the hash of the files in memory, before they are written to the disk by Orthanc. This information is safely stored inside the database for any incoming file attachment.

It is possible to ask Orthanc to check by itself whether some attachment file was corrupted (i.e. to check whether the MD5 hash stored in the database corresponds to the hash of the file on the disk):

$ curl -X POST http://localhost:8042/instances/f257b066-f3992cc4-ca6a5e5f-3f8dcf3a-d4958939/attachments/dicom/verify-md5 -d ''

This MD5 may be different if errors occurred while the DICOM file was initially written to the storage, or if the file contents were tampered with afterwards.

You can retrieve the stored MD5 hash of a DICOM instance as follows:

$ curl http://localhost:8042/instances/f257b066-f3992cc4-ca6a5e5f-3f8dcf3a-d4958939/attachments/dicom/md5

Windows-specific issues

  • Under Windows, Orthanc creates the “OrthancStorage” folder, and crashes with the error “SQLite: Unable to open the database”: Your directory name is either too long, or it contains special characters. Please try and run Orthanc in a folder with a simple name such as C:\Orthanc.

  • If you run Orthanc as a Windows service, and if you want to store the Orthanc database (or the Orthanc Web Viewer plugin) onto a shared network drive, you might encounter an error boost::filesystem::status: The specified server cannot perform the requested operation. This probably means that the SYSTEM account is not allowed to access the mapped network drive. The easiest approach to this end consists in configuring the Windows service to run as user (instead of the default SYSTEM account), as explained in this thread on the Orthanc Users forum (cf. additional reference elsewhere on Internet).

  • If Orthanc crashes when handling one large DICOM file, this most probably indicates a memory allocation error. Indeed, some precompiled official Windows binaries are compiled using a 32bit compiler. As a consequence, Orthanc only has access to less than 4GB of RAM. If this is an important limitation for you, precompiled command-line versions of Orthanc for Windows 64bit are also available.

  • Avoid installing Orthanc, its database or its storage area in folders whose names contain spaces or special characters.

  • If you run Orthanc as a Windows service, with the Python plugin enabled, you might have to change your PATH environment variable for Orthanc to find the Python DLL. The easiest approach to this end consists in configuring the Windows service to run as user (instead of the default SYSTEM account), as explained in this thread on the Orthanc Users forum (cf. additional reference elsewhere on Internet).